Antigenic variation of respiratory syncytial virus may play an important role in the development of disease in newborn seropositive infants and allow reinfection of older children to occur. Unlike influenza, the variation among RSV isolates from the same epidemic and between epidemic years is totally unpredictable. Although RSV isolates share conserved stable epitopes, little is known about the individual response to this virus following natural infection or immunization. Therefore, random genetic and antigenic heterogeneity among circulating RSV isolates may be an important obstacle to vaccine development. In addition, live attenuated viruses used as vaccines are susceptible to the same type of instability and production of these vaccines could result in virus strains with altered immunogenic properties when compared to the parent or seed virus. In order to make rapid comparisons we determined the sequence of the F gene of 15 RSV strains directly from cDNA amplified by Taq polymerase using the polymerase chain reaction (PCR). Nucleotide and deduced amino acid sequences were compared to sequences determined from M13 cloned cDNA. Direct sequencing of PCR amplified cDNA proved to be a reliable and accurate method for determining RSV gene sequences. This could be used to type RSV isolates, compare wild type versus vaccine strains of virus dring a clinical trial and also check lots of live attenuated vaccine for consistency.